1. Field of the Invention
This application relates to a rotary positive displacement pump, such as an internal or external gear pump, a lobe pump, a vane pump or a progressive cavity pump; and more particularly, relates to techniques for protection, e.g., for a dry run condition, for such a rotary positive displacement pump.
2. Brief Description of Related Art
Many different types or kinds of pumps and external protective devices, including rotary positive displacement pumps with external protection devices, are known in the art. By way of example, some known external protection device disadvantages associated with the same and set forth below:
One known device PMP 25, provided by a company named Load Controls, Inc. (Sturbridge, Mass.), uses a load monitor technique that provides pump protection by observing the motor amperage draw and speed and then correlating the resulting power reading to various operating conditions (e.g. dry running, closing valves). See U.S. Pat. Nos. 5,930,092 and 5,754,421, which are hereby incorporated by reference in their entirety. One disadvantage of this known device is that it is suitable only for constant speed applications and fails to distinguish control differentiation from various system upset conditions.
Another known device, provided by a company named ABB Industry Oy (Helsinki, Finland), uses a technique based on a variable frequency drive that has parameters that allow maximum and minimum torque values to be configured to prevent the load driver (motor) from operating outside of these parameters. One disadvantage of this variable frequency drive technique is that it does not provide logic for interpreting normal operating conditions from system upsets, such as distinguishing between a higher power requirement due to increased system resistance versus a higher torque condition caused by dry running.
Other known devices consist of flow or pressure switches or liquid presence/absence detectors to identify undesired operating conditions. However, the use of additional process flow or pressure switches adds cost and complexity to the drive system, a potential failure point, and unnecessary cost.
U.S. patent application Ser. No. 11/601,373, filed 17 Nov. 2006, entitled “Pump Protection Without the Use of Traditional Sensors,” by A. Stavale et al., which was published as U.S. 2007/0212229 A1 and is incorporated by reference in its entirety, sets forth techniques for providing pump protection for centrifugal pumps. Centrifugal pumps have a very different principle of operation than do rotary positive displacement pumps. In centrifugal pumps power varies as the cube of the speed change (FIG. 1) and torque varies as the square of the speed change. In addition, the tune process for dry run protection of centrifugal pumps described in patent application Ser. No. 11/601,373 is performed at a closed valve condition. The tune process for dry run protection of rotary positive displacement pumps could not be performed at the closed valve condition, since rotary positive displacement pumps will quickly destroy itself if operated at closed valve condition without intervention. For these reasons, the techniques disclosed in patent application Ser. No. 11/601,373 would not be applied to rotary positive displacement pumps.
None of the aforementioned patents or publications teach or suggest the technique described herein for providing pump protection for rotary positive displacement pumps, as set forth below.